In hydroponics, water/nutrients are generally kept in a large storage tank and recycled. This liquid is often heated through conduction or thermal heating by the sun. It must therefore generally be cooled before it can be applied to the plants. Also generally there can be a build up of noxious gases within the storage tank. If the storage tank is sealed, then those gases may be forced into solution. This gas may be deleterious to the plant life if absorbed.
Heat exchange apparatus, in general, are well known. In industrial processes, heat energy is transferred by a variety of methods, including conduction in electric-resistance heaters; conduction convection in exchanges, dwellers, and condensers; radiation in furnaces and radiant-heat dryers; and by special methods such as dielectric heating.
The design and testing of practical heat-exchange equipment are based on the general principles of heat transfer. In simple devices, the vital quantities such as average temperature difference and heat transfer coefficient can often be evaluated easily and with considerable accuracy, but in complex processing units evaluation may be difficult and subject to considerable uncertainty. The final design of heat exchange equipment is nearly always a compromise, based on engineering judgment, to give the best overall performance in the light of service requirements.
Sometimes the design is governed by considerations which have little to do with heat transfer, such as the space available for the equipment or the pressure drop which can be tolerated in the fluid streams.
Heat exchangers are so important and so widely used in the process or chemical industries that the principles of their design have been highly developed. Standards devised and accepted by the Tubular Exchanger Manufacturers Association (T. E. M. A.) are available covering in detail areas such as materials, methods of construction, technique of design, and dimensions for exchangers. Most exchangers are liquid to liquid heat exchangers, but gases and non-condensing vapours can also be treated in them.
Already known are tubular type exchangers, and also plate type exchangers. A tubular type exchanger generally has a first fluid flowing in tubes inside a larger fluid tight shell. A second fluid flows in the shell, outside the tubes, either cooling or heating the fluid flowing in the tubes. This heating or cooling is generally accomplished mainly by conduction from the hot fluid to the cooler fluid through the tube wall.
In plate type exchangers, metal plates, usually with corrugated faces, are supported in a frame; hot fluid passes between alternate pairs of plates, exchanging heat with the cold fluid in the adjacent spaces. The plates are typically approximately 5 mm apart. They can be readily separated for cleaning; additional area may be provided simply by adding more plates.
Other, more practical or readily available methods of heating or cooling are also known. One such method of heating is thermal heating. In this type of heating, fluids are stored in vessels, and the vessel is exposed to the sun. The heat energy from the sun heats the liquid inside the vessel.
Methods of cooling, similar in principle to thermal heating are also known. A simple example of evaporative cooling is known, particularly in off-road and long-distance trucking. In this method of cooling, a storage vessel is surrounded by cloth, the entire vessel and cloth then submerged in water, and attached to the front of a moving vehicle. Due to the speed of the moving vehicle and the air passing by the storage vessel, now surrounded with wet cloth, evaporation takes place. Due to the fact that evaporation requires heat energy to heat the water above a particular temperature, heat is absorbed from the water inside the storage vessel, thus cooling it.
The above methods of cooling and heating are not generally appropriate for small-scale hydroponics operations. Tubular type exchangers and plate type exchangers are very expensive and require large amounts of maintenance. For these reasons, they are often only found in large chemical plants. They are highly complex pieces of equipment and as such are not serviced easily by untrained operators. They require special knowledge and training which is not generally available to an ordinary user. They are generally suited only for large throughput situations.
Methods of the gas exchange are also known. Particularly mass-transfer operations known as gas absorption and stripping, or desorption are known.
In the gas absorption, a soluble vapour is absorbed from its mixture with an inert gas using a liquid in which the sought after gas is more or less soluble. The washing of ammonia from a mixture of ammonia and air by means of liquid water is a typical example. The solute gas is subsequently recovered from the liquid by distillation, and the absorbing liquid can either be discarded or reused. Sometimes a solute is removed from a liquid by bringing the liquid into contact with an inert gas; such an operation, the reverse of gas absorption, is called desorption or gas stripping.
The methods of gas exchange have disadvantages which are similar to the heat exchanger situation. They require special skills and training for operation of the required equipment and as such are not used by users without such training. They also are expensive pieces of equipment which are often quite large and complex and therefore are out of the budget of smaller users.
It simply would not be economically viable, nor practical, to use a conventional heat or gas exchanger, in a relatively small business such as a hydroponic primary producer. Often these large-scale pieces of equipment have their own problems, and as such would not meet the needs of a hydroponic grower.